1. Field
The present invention is concerned with a process for the catalytic hydrogenation of organic compounds on a catalyst of amorphous metal alloys under near-critical or supercritical conditions of the solvent.
2. Description
Amorphous metal alloys are also known in the art as metallic glasses. They can be produced, for example, by the shock cooling of the molten alloy on a cooled, rotating copper wheel. Depending on the nature of the surface of the copper wheel and on the precise process conditions there are obtained thin metal ribbons (in the case of a smooth copper surface) or flake-like particles, so-called flakes, (in the case of a rough copper surface). The thickness of the metal ribbons or of the flakes lies in the range between 5 and 50 .mu.m, preferably between 10 and 30 .mu.m, especially about 20 .mu.m. If the ribbons or flakes were to be made thicker, heat cannot dissipate at a rapid enough rate. Increasing the thickness increases the formation of a crystalline alloy structure.
Metallic glasses obtained in the above process are non-porous. Their surface is therefore formed exclusively by their external geometric surface. This surface area is only about 0.5 m.sup.2 /g in the case of the preferred metal ribbons or flakes of 20 .mu.m thickness. In contrast, activated nickel powder catalysts have specific metal surface areas of more than 100 m.sup.2 /g and finely dispersed palladium catalysts carried on active charcoal have specific metal surface areas of more than 40 m.sup.2 /g.
Although the catalytic properties of metallic glasses have been investigated intensively, because of their small metal surface area, they have never before been used as catalysts for industrial processes. An overview on the state of the art for catalysis on metallic glasses is found in the article "Glassy Metals in Catalysis" by A. Baiker in "Topics in Applied Physics" Vol. 72., pages 121-162, Ed. H. Beck and H.-J. Guntherodt, published by Springer 1994.
Metallic glasses can catalyze a large number of hydrogenation reactions, for example the hydrogenation of nitrogen, ethene or butadiene (see European Patent No. 0 173 088 B1) at temperature in the range of 90.degree. C. to 450.degree. C. and gas pressures of below 10 bar.
Catalytic activity depends on the respective alloy composition of the metallic glass. For example, Pd.sub.81 Si.sub.19 glasses have been found to be especially suitable for the selective hydrogenation of alkynes to alkenes [A. Molnar et al.; J. Catal. 101: 67-72 (1986)]. However, these hydrogenations were carried out at room temperature and at atmospheric hydrogen pressure.
In spite of the fundamental suitability of metallic glasses for catalytic hydrogenations, the small metal surface, low thermal stability and the formation of stable oxide layers on the surface of the metallic glasses during their production were seen to be serious obstacles for their use in industrial hydrogenation processes [D. Gasser and A. Baiker, Applied Catalysis, 48: 279-294 (1989)]. Moreover, it was suspected that the action of hydrogen would cause the metallic glasses to become friable, which would finally lead to their pulverization.
Thus, an object of the present invention is to provide an industrial process for the catalytic hydrogenation of organic compounds on metallic glasses, which overcomes the obstacles known from the state of the art for the use of metallic glasses in industrial hydrogenation processes, permits hydrogenations with high space-time yields and good long-term stability of the catalytic activity, and solves the problems associated with the reactant spending too much time on the catalyst.